What happens with constraints when a view is removed

问题

The question I have is simple but I couldn't find any information in the documentation.

What happens with layout constraints when a view is removed from the view hierarchy (or moved to another view)?

For example, let's have container C with subviews A and B. Container C holds some constraints. Then we call [A removeFromSuperview]. What happens with the constraints for A?

What then happens if we add A to C again?

回答1:

The constraints are removed. If you add A again, you will have to make new constraints for it, or if you save the constraints before you remove A, you can add them back. When I do something like this, I save the constraints like this for a view called view1:

self.portraitConstraints = [NSMutableArray new];
for (NSLayoutConstraint *con in self.view.constraints) {
    if (con.firstItem == self.view1 || con.secondItem == self.view1) {
       [self.portraitConstraints addObject:con];
    }
}

回答2:

Since I had this question too, I checked the Apple Docs just for kicks, and it turns out that it is documented that the constraints are removed.

The documentation for the UIView removeFromSuperview method states:

Calling this method removes any constraints that refer to the view you are removing, or that refer to any view in the subtree of the view you are removing.

I'm not sure if this was documented last year when the original question was posted, but I just thought I'd share this information in case anyone needed it...

回答3:

Be aware though, that if you have two independent parent views A and B, and a subview C, where C is currently a subview of A, with appropriate constraints, that calling [B addSubview:C] will NOT clear any constraints relating to A and C, and auto layout will start throwing exceptions, because those constraints no longer relate to views in the same hierarchy.

You will need to call [C removeFromSuperview] explicitly to remove the constraints, before adding C to B.

This is true on Mac OS X - I haven't checked iOS

回答4:

The constraints are also removed when you [A removeFromSuperview]

They are forgotten and adding A to C again adds no constraints.

回答5:

They are removed too, you can do a simple test. Pick up a view SUBVIEW and create costraints that constraint SUBVIEW to follow its superview resizing (like attched to to superview edges). To do that you add SUBVIEW as a subview to this CONTAINERVIEW and add as constraints something like that:
V:|-[SUBVIEW]-|
H:|-[SUBVIEW]-|
These constraints should be added to SUBVIEW superview, thus CONTAINERVIEW.
If you remove SUBVIEW by simply checking all the CONTAINERVIEW constraints you could see that two aren't around anymore.

回答6:

This question also can be proved by interface builder. When drag and drop a UIView on the ViewController add constraints then remove the UIView, you can see the blue constraints disappear.

来源：https://stackoverflow.com/questions/18617991/what-happens-with-constraints-when-a-view-is-removed

标签

ios

autolayout

nslayoutconstraint